1. Field of the Invention
The present invention relates to a method for evaluating the quality of a read signal when optical recording and reading is performed using a laser beam and to a method for optical recording and reading by means of the quality evaluation method.
2. Description of the Related Art
Conventionally, optical recording mediums such as CD-DAs, CD-ROMs, CD-Rs, CD-RWs, DVD-ROMs, DVD-Rs, DVD+/−RWs, and DVD-RAMs are widely used for viewing digital video and for recording digital data. The recording capacity required for such optical recording medium has been increasing every year. To meet this requirement, the so-called next generation DVDs capable of storing large video and data files have begun to be commercialized. In such next generation DVDs, the increase in recording capacity is achieved by reducing the wavelength of a laser beam used for recording and reading to 405 nm.
For example, in the Blu-ray Disc (BD) standard, which is one of the next generation DVD standards, the numerical aperture of an objective lens is set to 0.85, whereby as much as 25 GB of data can be recorded in and read from one recording layer. However, it is expected that the size of video and computer data files will further increase in the future. Accordingly, it is contemplated to increase the capacity of a disc by reducing the size of recording marks so that the linear recording density in each layer is increased.
Meanwhile, under optical recording and reading conditions in which a laser beam having a wavelength of λ and an objective lens having a numerical aperture of NA are used, a so-called resolution limit exists. Specifically, when a encoded signal contains a sequence of a recording mark and a space each having a size equal to or less than 1.0×λ/(4×NA), the amplitude of the read signal from the mark-space sequence is substantially zero. In the current standards of the above CD, DVD, and BD, the size of the minimum recording mark in a encoded signal is greater than 1.0×λ/(4×NA), and therefore the resolution limit is not reached. Hence, an amplitude sufficient for reading a signal can be obtained for any combination of recording marks and spaces by using an appropriate equalizer. Therefore, by slicing a read signal at a certain voltage level, the quality of the record and read signal can be evaluated according to the information of the position of the intersection of the slice level and the amplitude curve of the read signal (or to edge jitter).
According to the studies of the present inventors, when the size of recording marks is reduced such that the minimum size is equal to or less than 1.1×λ/(4×NA), the amplitude of a signal from a sequence of a recording mark and a space each having the minimum size is below a practically acceptable level. Moreover, when the size of the recording marks is reduced to 1.0×λ/(4×NA) or less, the amplitude becomes identically zero due to the resolution limit, so that the signal evaluation using the edge jitter cannot be made.
As a technique for avoiding such a problem, there is a PRML (Partial Response Maximum Likelihood) detection method in which a PR equalizer and an ML decoder (such as a Viterbi decoder) are used. The PR equalizer has a function of correcting an actual read signal to match the corrected signal to a reference PR characteristic. A coefficient used for the correction is called an equalization coefficient, and a plurality of equalization coefficients corresponding to different amplitudes of the read signal are used in the PR equalizer.
In the PRML detection method, when a PR (1, 2, 1) characteristic with a constraint length of 3, for example, is used, an impulse response from an actual recorded bit is represented by a sequence having an amplitude of PR (h1, h2, h3). Therefore, in the PR equalizer, the equalization coefficients are used to match the read signal having an amplitude of PR (h1, h2, h3) to the reference PR (1, 2, 1) characteristic, whereby noise components are eliminated.
The ML decoder computes the deviation of the signal sequence equalized by the PR equalizer from each of all possible ideal responses and selects one ideal response having a minimum cumulative deviation (this ideal response is referred to as a maximum likelihood ideal response). An identified signal is obtained from the maximum likelihood ideal response. In this manner, a correct identified signal can be extracted even when the read signal has a small amplitude and is embedded in noise.
The characteristics of individual optical recording mediums differ from each other, and also the characteristics of optical heads of individual reading devices differ from each other. In order to cope with the differences, the equalization coefficients used in the PR equalizer are adjusted, or an optimal PR equalizer is selected. In addition, an optimal decoder is selected as the ML decoder.
As has been described, the PRML detection method is basically a reading technique. Therefore, the error rate of a signal obtained using the PRML detection method largely depends on, in addition to the recording medium used and the recording strategy used, the types of signal processing techniques such as PR equalization and ML decoding. The linear recording density in each layer will be increased in the future. In such a circumstance, various combinations of PR equalization and ML decoding methods can be used. Therefore, it is quite difficult to select an appropriate PRML detection method according to a unified standard or the like.
Meanwhile, even in a recording and reading system in which the PRML detection method is used, it is highly necessary to reasonably and objectively evaluate the level of the quality of a record and read signal. However, as has been described, the selection of the PR equalization and ML decoding methods used in the PRML detection method depends on the discretion of individual manufacturers. Therefore, the level of the quality of a record and read signal depends on the selected PR equalization and ML decoding methods, and therefore the objectivity of the quality level is not guaranteed. Specifically, the error rate of an identified signal obtained using the selected PRML detection method is an evaluated value that is effective only for the system used, and therefore the quality of the optical recording medium itself and the suitability of the selected recording strategy cannot be determined from the error rate. In addition, in the recording and reading device, the suitability of the combination of the provided optical recording medium and the selected recording strategy cannot be determined only from the error rate.